332 CHILD— AGE CYCLES AND 



of smaller size than the individual from which the piece was taken, 

 the size of the new individual depending on the size of the piece. 

 In undergoing this reconstitution the piece uses up a part of its own 

 body substance as a source of energy and of the new cell material 

 formed. 



A test of the metabolic condition of the new individual thus 

 formed shows that it is physiologically younger than the animal 

 from which the piece was taken, and the smaller the fraction of 

 the original body from which it developed, the younger it is. By 

 feeding it can be made to grow, to go through the life history and 

 to become old again, and may then again be cut into pieces which 

 once more become young individuals, and this process can be car- 

 ried on for a long time, probably indefinitely. I have carried an 

 experiment of this sort through twenty generations. Evidently 

 the process of reconstitution of these pieces into new animals in 

 some way brings about a greater or less degree of rejuvenescence. 



Another experiment affords a clue as to the manner in which 

 this rejuvenescence occurs. If the full-grown planarians which 

 are 25-30 mm. in length are deprived of food, they do not die of 

 starvation in a few days, but gradually become smaller, and can be 

 reduced to a length of a millimeter or less before death occurs. 

 Tests of metabolic condition show that during this reduction the 

 metabolic rate is increasing, the animals are growing younger. 

 Morphologically also they show indications of rejuvenescence. By 

 feeding we can stop this process of rejuvenescence at any time and 

 make the animals begin to grow old again, and we can repeat this 

 process again and again. 



In another species of planarian in which the length of the life 

 cycle under ordinary conditions in nature is about two months, I 

 have been able to keep the same individual worms alive and in 

 essentially the same physiological condition for a much longer 

 period than the usual length of life, simply by regulating quantity 

 and quality of food, i. e., the animals were fed just enough of the 

 food, found by experience to be most suitable, to prevent reduc- 

 tion in size and not enough to permit growth. With this procedure 

 they remained in practically the same physiological condition, of 



